One method is, for example, disclosed in U.S. Pat. No. 5,291,270. This method is especially provided to determine edges and bores of workpieces to be measured such as parts of a vehicle chassis. For this purpose, an optical triangulation probe is suggested in this patent which measures the spacing of the probe to the workpiece surface and moves over the edge to be measured for measuring a point thereof. If a sudden jump results in the measured distance values, then this indicates that the optical probe passed over an edge at this point in time so that the position of a point of the edge can be determined from the corresponding distance values and the machine positions.
The optical probe is suspended from a mechanism of a coordinate measuring apparatus in order to be able to suitably move the optical probe over the surface of the workpiece. With the mechanism, the optical probe can be moved in three mutually perpendicular directions. Additionally, the optical probe is connected to the mechanism via a rotation-pivot unit so that the optical probe can be additionally rotated about two mutually perpendicular axes. In this way, the scanning beam of the probe can always be suitably adjusted to the surface of the workpiece to be measured.
What is distinctive about this method is that the accuracy of the determined point of the edge is dependent upon how close the sequentially recorded scanning points lie next to each other. The optical probe must record measuring points lying very close to each other in order to achieve a high accuracy. The central computer of the coordinate measuring apparatus assumes the entire evaluation of the measured values recorded by the probe. For this reason, only a relatively low measuring speed is possible for high measuring accuracy. Furthermore, nonuniformities in the edge can greatly alter the measuring result. If, for example, the path of the scanning point is guided over a location at which a nonuniformity in the form of a ridge is located, then this can greatly alter the result of the measured point of the edge.
From the state of the art, so-called linear probes are known which are utilized in a manner similar to the triangulation probe known from U.S. Pat. No. 5,291,270. The difference in this linear probe compared to the triangulation probe is that the scanning point in the linear probe is continuously moved back and forth along a straight line. The detection of the edge then takes place in that the scanning point of the linear probe is guided perpendicularly over the edge in its linear direction. Similarly, the edge is detected in that an abrupt change is detected for the first time in the scanned distance values.
The same characteristics apply to the disclosed linear probe as applied to the triangulation probe disclosed in U.S. Pat. No. 5,291,270.
Furthermore, a probe is known from German patent publication 2,405,102 with which edges can likewise be measured. The probe is here so configured that a light beam is guided on a circular path over the edge of the workpiece to be measured. A photodetector is mounted rearward on the opposite lying end of the workpiece. The intercept point of the circular path of the light point with the edge, and therefore the position of the edge, can be determined from the measured brightness and darkness times of the photodetector.
The characteristics of the method described in German patent publication 2,405,102 apply as already explained in connection with U.S. Pat. No. 5,291,270.